Laser illumination systems are commonly used in projection displays to provide high power illumination and saturated color. However, while lasers provide bright images with good color, the image quality can be degraded due to speckle.
Speckle arises when coherent light is reflected from a rough or dusty surface, or propagates through a medium with random refractive index variations. More specifically, it arises when the reflected light, which includes multiple beams with differential delays greater than the wavelength of light, interfere at the detector (e.g. human eye, or square law photodetector). This interference provides an uneven, random, fluctuation of light intensity commonly referred to as a speckle pattern.
In projection displays, speckle generally originates when the light reflects off of the display screen, which typically has a surface roughness greater than one quarter of the wavelength of the laser light. The resulting random spatial interference of the reflected laser light produces a speckle pattern that significantly degrades the quality of the image (e.g., making it appear grainy and/or less sharp). In addition, depending on the view point, the speckle pattern may change due to the different characteristics of relative phase delays in a given direction. As a result, the image being observed changes with view point and the optical system fails to reliably recreate a high fidelity image.
Speckle is typically quantified by the speckle contrast. The prior art describes various techniques for reducing speckle and/or reducing the speckle contrast. For example, one approach has been to increase the number of longitudinal modes of the laser such that speckle patterns from multiple wavelengths average to a smooth profile. Another approach has been to tile an array of coherent laser diodes (LD) to provide spatially incoherent illumination. Unfortunately, this approach is expensive, and is not always practical, since many miniature projectors rely on a single LD chip to output tens of lumens illumination. Yet another approach has been to create polarization diversity in the laser illumination. For example, one laser beam can be split into two polarizations, with the first polarization being allowed through a polarization beam splitter (PBS) and the second polarization delayed by greater than the coherence length of the laser (e.g., see U.S. Pat. Nos. 3,633,999 and 4,511,220). Generally, this approach is bulky and has limited speckle contrast reduction. In addition, it is not ideal if the laser coherence length is very long.
In addition to changing the LD arrangement (spatial) or manipulating the laser device characteristics (polarization and longitudinal modes) to reduce the spatial and temporal coherence of the laser beam, another approach has been to create many varied boiling speckle patterns that allow for temporal averaging (e.g., by the human eye or photodetector) to reduce intensity non-uniformity. For example, one approach is to vibrate the display screen. Unfortunately, for a large projection screen, this is not very practical. Accordingly, the more common approach is to use an external optical element, such as a diffuser (e.g., see J. W. Goodman et al., “Speckle reduction by a moving diffuser in laser projection displays,” Annual Meeting of the Optical Society of America, Rhode Island, 2000), a phase plate (e.g., see U.S. Pat. Nos. 6,323,984 and 6,747,781), or a random diffractive optical element (e.g., see L. Wang et al., Speckle reduction in laser projection systems by diffractive optical element,” Appl. Opt. 37, pp. 177-1775, 1998), which is vibrated or spun to yield multiple phase delays over time. In another approach, an ultrasonic modulator is used to shift the interference fringes.
While the more commonly used methods of creating many varied boiling speckle patterns to provide temporal averaging have been reasonably successful at reducing spectral contrast, they have been generally limited by the surface pattern physically etched/embossed in prior art diffusers/phase plates. For example, these raised surface patterns have been shown to significantly degrade the quality of the laser beam.